An adjustable-frequency NMR probe is known from EP 0 930 511 B 1.
NMR (=nuclear magnetic resonance) spectroscopy is an efficient instrumental analysis method with which it is possible to study the chemical composition of measurement substances (samples). In the process, the measurement substance is exposed to a strong, static magnetic field, resulting in nuclear spin alignment in the measurement substance. After RF (=radio-frequency) pulses have been radiated in, RF signals emitted by the measurement substance are recorded and are evaluated in order to determine the chemical composition.
The strong, static magnetic field is often generated by superconducting magnets which are cooled in a cryostat. The sample to be studied is arranged in a room temperature bore of the cryostat. The RF pulses are radiated in and the RF signals are received by one or more RF resonator coils of a probe that projects into the room temperature bore. The probe typically also holds the sample to be measured.
The arrangement in the room temperature bore of the cryostat means that the installation space for the probe is extremely limited. In particular, the probe usually has to conform to an essentially cylindrical design.
The frequency of the RF pulses emitted by an RF resonator coil and the RF signals recorded thereby depends on the type of atomic nuclei studied and the strength of the static magnetic field. The RF resonator coil is part of an RF resonant circuit (electrical circuit), the resonant frequency of which results from the components contained (in particular the capacitance and inductance thereof) and the way said components are connected. The resonant frequency has to be matched to the type of atomic nuclei studied and the strength of the static magnetic field.
For NMR experiments involving different types of atomic nuclei and/or different magnetic field strengths, different probes may be made available, but this is very expensive and causes significant complexity when making the modifications.
EP 0 930 511 B1 relates to probes that can be set to different frequencies. However, the limited space restricts the number of components and the adjustment mechanisms thereof. An NMR probe is proposed comprising a capacitor having an adjustable capacitance, an inner capacitor element being movable relative to an outer capacitor element, using a screw connection, in order to allow for a compact structure. In this structure formed in the manner of a cylindrical capacitor, the capacitance can be adjusted steplessly, but only in a small capacitance range.
DE 198 33 350 C1 discloses a probe in which a measuring coil is connected to different input terminals in order to feed low-frequency isotopes (X) and the 1H core at a higher frequency. The 1H frequency is decoupled using a series capacitor and a lambda/4 line that has a capacitance per unit length that can be adjusted, over the length of the line, using a shorting plunger. The adjustment range is very small in this case too.
A lambda/4 line for an NMR probe that can be adjusted in a similar manner is also known from EP 1 651 973 B1.
In broadband probes by Bruker BioSpin AG, Fallanden, CH, a plurality of shift registers are installed in the probe. One capacitor can be selected by each shift register; the selected capacitors of the different shift registers are connected in parallel. The resonant frequency of an RF resonator coil can be adjusted by appropriately adjusting the shift register and/or selecting the capacitors. However, this design is expensive, in particular due to the actuators required for each shift register, and requires a lot of installation space, which limits the performance of the probe.